Global notifications and expirations.

This adds a 'global' flag for notifications as well as an 'expires' timestamp.

This will allow for easy creation of global notifications that also have an expiration date.  It should be useful for system-wide notification where there is no specific list of recipients.  It addresses the issues discussed in #124

Added specs around the setting of the expiration date, but global is just a new column so no specs added there.  Let me know if you'd like some.